So I was asked "how do you calculate the GPA? My transcripts only have percentages mentioned."
I also had a similar problem. In Brazil, there is a variety of grading systems, depending on the school. The system used in my bachelor's transcript was different from my MBA's, which was different from my Master's...
And my bachelor's GPA does not seem great, if you don't know that at my school it was impossible to get a GPA equivalent to a 3.8-4.0, for example. By the way, I explained it in my statement of purpose.
So, how do you calculate the GPA? It depends on the school you're applying to. There was one school who informed at its website how you should make the conversion. There were schools who asked you to not convert at all, but inform them the grade you actually got (be it in letters, percentages, or other numbers) and what is the grade system used by the school. There were schools which asked me about explanations and additional documents after I sent my application, because they were in doubt when reviewing my application. And there were schools which did not inform anything at all about how you should make the conversion. For those cases, I used the WES Calculator, since WES is an organization they usually know about.